Fixed-field per-ion Curie coefficient
= Fixed-field per-ion Curie coefficient
{title2=$C_\mu=(g\mu_B)^2J(J+1)B/(3k_B)$}
For independent localized angular-momentum-$J$ ions at fixed field, the weak-polarization mean moment is $\langle\mu_z\rangle=C_\mu/T$. Unlike the volume-susceptibility <Curie constant>, this coefficient includes the magnetic field and no ion number density. The expansion requires $g\mu_B|B|J\ll k_BT$; the moment saturates toward $g\mu_BJ$ beyond that regime.